1. Field of the Invention
The present invention relates to an optical disc apparatus such as an optical information recording and reproducing apparatus or optical information reproducing apparatus for recording information on an optical disc, such as a photo magnetic disc, and/or reproducing the recorded information, and more specifically it relates to a laser adjusting apparatus thereof.
2. Description of Related Art
In a known optical disc apparatus, an objective lens which converges a laser beam from a laser source onto an optical disc, and/or receives light reflected by the optical disc, is linearly moved back and forth in the radial direction across the tracks of the optical disc. In such an optical disc apparatus, light from the laser source is made parallel with the direction of movement of the objective lens in a vertical plane lying in the radial direction of the optical disc by a galvanomirror unit. The galvanomirror unit varies the angle of the galvanomirror to change the direction of light reflected thereby, so that the reflected laser is made parallel with the linear direction of movement of the objective lens.
FIG. 8 shows a known optical disc apparatus including a galvanomirror unit, in which light (laser beam) from a laser source 1 is collimated by a collimating lens 2 and is shaped into a circular light flux by first and second beam shaping prisms 3 and 4, respectively. The shaped light is then reflected by a galvanomirror unit 5' to be made incident on an objective lens 8, opposed to optical disc 9, which is moved on and along guide rails 10 together with a movable mirror 7. The galvanomirror unit 5' is rotatable about a horizontal shaft 5a, so that the direction of light reflected thereby can be made parallel with the linear direction of movement of the objective lens 8, i.e., the lengthwise direction of the guide rails 10, by adjusting the angle of the galvanomirror 5'. The direction of the laser is adjusted in the vertical plane lying in the radial direction of the optical disc 9 by the galvanomirror unit 5'. Note that the galvanomirror unit 5' can not adjust the direction of the laser in a horizontal plane.
In the known optical system as mentioned above, the deflection angle of the laser beam reflected by the galvanomirror unit 5' is equal to 2 times the angular displacement (angle) of the galvanomirror unit 5'. Consequently, the velocity "v" of the laser beam moved in the radial direction of the optical disc 9 through the objective lens 8 by the galvanomirror unit 5' is given by: EQU v=2 f .beta.
wherein f designates the focal length of the objective lens 8 and .beta. the rotational angular velocity of the galvanomirror unit 5'.
The focal length of the objective lens 8 is usually fixed at a predetermined value, and accordingly, it is necessary to move the galvanomirror unit 5' at high speed to effect a quick tracking control. However, since the galvanomirror unit 5' is relatively heavy, it is very difficult to control high speed rotation and quick stopping of the galvanomirror unit 5' due to its inertia. This limits the speed of the tracking control.